Gas sensor

Abstract

A sensor element includes: a main pump cell including an inner pump electrode facing a first inner space into which a measurement gas is introduced, an external pump electrode provided on an element surface, and a solid electrolyte therebetween; an auxiliary pump cell including an auxiliary pump electrode provided facing a second inner space, the external pump electrode, and the solid electrolyte therebetween; and a measurement pump cell including a measurement electrode, the external pump electrode, and the solid electrolyte therebetween. The inner pump electrode has a porosity of 10-25%, the auxiliary pump electrode has a porosity of 30-50%, a thickness ratio of both the electrodes is 1.0-4.0, and current flowing to the main pump cell has a current density of 0.05-0.5 mA/mm 2 when the measurement gas has an oxygen concentration of 20.5%.

What is claimed is: 
     
         1 . A limiting current type gas sensor comprising a sensor element made of an oxygen-ion conductive solid electrolyte and capable of specifying the concentration of NOx in a measurement gas, said sensor element comprising:
 a gas inlet through which the measurement gas is introduced from an external space;   a first inner space communicated with said gas inlet under a predetermined diffusion resistance;   a second inner space communicated with said first inner space under a predetermined diffusion resistance;   a main pump cell as an electrochemical pump cell including an inner pump electrode provided facing said first inner space, an external pump electrode provided on a surface of said sensor element, and said solid electrolyte provided between said inner pump electrode and said external pump electrode;   an auxiliary pump cell as an electrochemical pump cell including an auxiliary pump electrode provided facing said second inner space, said external pump electrode, and said solid electrolyte provided between said auxiliary pump electrode and said external pump electrode;   a measurement electrode disposed inside said sensor element and interposing at least a diffusion limiting part between said measurement electrode and said second inner space; and   a measurement pump cell as an electrochemical pump cell including said measurement electrode, said external pump electrode, and said solid electrolyte provided between said measurement electrode and said external pump electrode, wherein   said inner pump electrode has a porosity P 1  of 10% to 25%,   said auxiliary pump electrode has a porosity P 2  of 30% to 50%,   a ratio T1/T2 of a thickness T1 of said inner pump electrode to a thickness T2 of said auxiliary pump electrode is 1.0 to 4.0, and   said gas sensor is configured and disposed so that current flowing to said main pump cell has a current density of 0.05 mA/mm 2  to 0.5 mA/mm 2  when said measurement gas has an oxygen concentration of 20.5%.   
     
     
         2 . The gas sensor according to  claim 1 , wherein said sensor element further includes:
 an air introduction layer to which air is introduced as a reference gas from outside of said sensor element;   a reference electrode covered by said air introduction layer;   a main pump control sensor cell as an electrochemical sensor cell including said inner pump electrode, said reference electrode, and said solid electrolyte provided between said inner pump electrode and said reference electrode;   an auxiliary pump control sensor cell as an electrochemical sensor cell including said auxiliary pump electrode, said reference electrode, and said solid electrolyte provided between said auxiliary pump electrode and said reference electrode; and   a measurement pump control sensor cell as an electrochemical sensor cell including said measurement electrode, said reference electrode, and said solid electrolyte provided between said measurement electrode and said reference electrode, wherein   said main pump cell is configured and disposed to pump out oxygen in said measurement gas in said first inner space by applying, between said inner pump electrode and said external pump electrode, said main pump voltage in accordance with electromotive force generated between said inner pump electrode and said reference electrode in said main pump control sensor cell,   said auxiliary pump cell is configured and disposed to pump out oxygen in said measurement gas introduced into said second inner space by applying, between said auxiliary pump electrode and said external pump electrode, pump voltage in accordance with electromotive force generated between said auxiliary pump electrode and said reference electrode in said auxiliary pump control sensor cell so that said measurement gas having an oxygen partial pressure thus made to be lower than in said first inner space reaches said measurement electrode,   said measurement pump cell is configured and disposed to pump out oxygen generated at said measurement electrode by applying, between said measurement electrode and said external pump electrode, pump voltage in accordance with electromotive force generated between said measurement electrode and said reference electrode in said measurement pump control sensor cell, and   a porosity difference P 2 -P 1  between said inner pump electrode and said auxiliary pump electrode is 30% or smaller.   
     
     
         3 . The gas sensor according to  claim 1 , wherein said inner pump electrode and said auxiliary pump electrode have a thickness of 5 μm to 30 μm and an area of 5 mm 2  to 20 mm 2 . 
     
     
         4 . The gas sensor according to  claim 2 , wherein said inner pump electrode and said auxiliary pump electrode have a thickness of 5 μm to 30 μm and an area of 5 mm 2  to 20 mm 2 .